Tissue elasticity is widely held as being strongly associated with disease, especially breast malignancy given the prominent place that physical examination holds in breast cancer screening. As a result, a sizeable technology effort has emerged to develop and evaluate new concepts for measuring and imaging parameters related to the mechanical characteristics of breast tissue. Project I, Magnetic Resonance Elastography (MRE), exploits phase contrast techniques to encode induced harmonic mechanical wave motion into phase accumulations during customized gradients to map the full displacement field in three dimensions from which mechanical property estimates are derived at or near the MR acquisition resolution. During the current funding period, Project I has successfully realized a robust MRE technique which has been deployed on a limited basis during clinical breast exams to demonstrate feasibility, provide preliminary estimates of the mechanical properties of normal breast, and highlight opportunities for extracting more complex mechanical behaviors. The existing MRE method is ready for systematic clinical deployment and will be used in a research plan consisting of (a) clinical evaluation, (b) continued technical advancement and (c) histopathological correlation to begin to identify the biological signatures of mechanical property contrast. The specific aims for continuation of Project I include (1) development of MRE driving and reconstruction techniques to estimate viscoelastic mechanical behavior in breast tissue, (2) Optimization of image acquisition sequences and methods to reduce exam time (3) exploration of shear modulus and viscoelastic property correlations with histopathological analyses in mastectomy specimens where adequate tissue is available to spatially sample and orient the imaged volume, and (4) participation in the clinical study designs executed by the Clinical Core targeting screening abnormalities recommended for biopsy, palpable masses on clinical breast exams and pilot exams of locally-advanced disease receiving neoadjuvant therapy. If successful, these aims are expected to generate evidence sufficient to estimate convincingly the potential of MRE to contribute to differential diagnosis, and pilot data in support of a role in treatment prognosis and therapy monitoring which will inform decisions on the initiation of larger clinical trials with MRE in the future.